Band engineering of Dirac cones in iron chalcogenides

By band engineering the iron chalcogenide Fe(Se,Te) via ab initio calculations, we search for topological surface states and realizations of Majorana bound states. Proposed topological states are expected to occur for nonstoichiometric compositions on a surface Dirac cone where issues like disorder scattering and charge transfer between relevant electronic states have to be addressed. However, this surface Dirac cone is well above the Fermi level. Our goal is to theoretically design a substituted crystal in which the surface Dirac cone is shifted toward the Fermi level by modifying the bulk material without disturbing the surface. Going beyond conventional density functional theory, we apply the Blackman, Esterling, and Berk coherent potential approximation in a mixed basis pseudopotential framework to scan the substitutional phase space of cosubstitutions on the Se sites. We have identified iodine as a promising candidate for intrinsic doping. Our specific proposal is that FeSe0.325I0.175Te0.5 is a very likely candidate to exhibit a Dirac cone right at the Fermi energy without inducing strong disorder scattering.